EFFECT OF LEFT VENTRICULAR VALVE PLANE SELECTION ON COMPUTATION OF LEFT VENTRICULAR EJECTION FRACTION FROM GATED THALLIUM SPECT: Computation of left ventricular ejection fraction (LVEF) from gated thallium SPECT studies requires the identification of valve plane to define the basal limit of the LV. In patients with coronary artery disease undergoing gated thallium studies using a 3-headed SPECT camera, we determined 1) the intra-observer variability of valve plane selection in 17 patients, and 2) the effect of variable valve plane selection on the calculated LVEF in 32 patients. For each patient, the LV was divided into 4 long axis tomograms encompassing the entire LV. Acquisitions were gated for 10 frames per cardiac cycle. Because thallium SPECT images are noisy, the endocardial border of the myocardium is often difficult to assess. To overcome this limitation, we recently developed and validated an algorithm for the computation of LVEF based on the epicardial contour of the myocardium. When valve plane selection was made by the same observer on each of the 10 frames for the 4 long axis tomograms (totalling 40 gated images per patient), on two separate days, the intra-observer reproducibility of LVEF was excellent (r=0.94, slope=1.03, y-intercept=0.48, SEE 6.25). Because gated thallium images are noisy and count poor, summing the 10 frames per cardiac cycle for each of the 4 long axis tomograms (totalling 4 summed gated images per patient) resulted in a significantly higher counts per image and clearer definition of the endo- and epicardial myocardial contour. However, valve plane selection from the 4 summed gated images provided variable LVEF values when compared to LVEF computed from the 40 gated images in the same patients (r=0.76, slope= 0.75, y-intercept=-3.56, SEE= 8.97). Because the myocardium shortens along the longitudinal as well as the short axis, selection of valve plane on the 4 summed gated images does not take into account contraction of LV along the vertical axis. This may explain the observed underestimation of LVEF by selecting valve planes on the 4 summed gated images compared with the 40 gated images. Thus, despite obtaining clearer definition of the endo- and epicardial myocardial contour by summing the gated thallium SPECT images, valve plane selection from summed tomograms adversely effects the computation of LVEF.PHS 6040 (Rev. 5/92).